In the fabrication of semiconductors there is a need to control the process parameters to ensure process consistency and repeatability. The need for process control is becoming more important as semiconductor devices are requiring sub-nanometer accuracy in dimension tolerance (e.g., CD, thickness, etch rate, uniformity, profile, etc.) as device nodes advance to smaller and smaller features (e.g., 90 nm and smaller). In the more advanced processes, feature size variations and dimensional tolerance in the fabricated device (within a wafer, wafer-to-wafer, lot-to-lot, die-to-die, chamber-to-chamber, etc.) resulting from all process variations are required to be smaller than 5 nm and within three-sigma of standard deviation. Soon, as wafer processing become even more advanced, the allowable feature size variations will be even smaller, e.g., smaller than 2 nm and within three-sigma of standard deviation.
One of the more difficult process parameters to control, maintain, and characterize is the process temperature. For example, the process temperature on chamber interior wall surfaces, the substrate support surface, and the substrate surface are difficult to control, maintain, and characterize. As discussed within the scope of the present invention, the reference to wafers and substrates is interchangeable since those of ordinary skill in semiconductor fabrication often interchangeably use both terms. Many process recipes are sensitive to process temperature variation. Temperature variation as small as 1 degree Celsius can have significant effect on the outcome of the process recipe. For example, in a semiconductor fabrication etch process, poly gate CD (critical dimension) can change by as much as 1 nm per 1 degree Celsius variation in the process temperature, e.g., the temperature on the surface of a substrate support, the surface of the substrate, etc. Some process recipes can be affected by even smaller variations, e.g., 0.5 degree Celsius, in the process temperature. Accordingly, accurate temperature control and measurement are becoming critical process control requirements as device nodes advance to smaller and smaller feature size. Therefore, accurate temperature measurement and characterization capable of measuring absolute temperature and sensing small temperature changes, e.g., changes as small as 0.5 degree Celsius or smaller, is desired.
Many of the currently available temperature measurement techniques have performance limitations and undesirable effects. For example, many of the currently available temperature measurement techniques are unable to measure the in situ process temperature accurately. The techniques that have the capability to measure the in situ process temperature of a wafer usually include placing a special wafer with temperature sensors embedded on the wafer. Placing a special wafer in the process chamber requires interrupting the normal flow of processing. Usually, placing a special wafer into the process chamber requires venting the process chamber to ambient pressure. Once the process chamber is vented, considerable amount of time is required to bring the chamber back to process operating conditions (e.g., pressure, temperature, etc.), which affects the throughput of lot processing. Also, in many cases, the embedded sensors could be a source of contamination that may cause device defects. In addition, these special wafers with embedded sensors are expensive and the sensors are typically not very robust. When the sensors are exposed to process operating conditions, they could fail or work improperly. Furthermore, the embedded sensors are not acceptable for delivering the desired measurement accuracy as most of these sensors have a temperature measurement uncertainty of 0.5 degree Celsius or more. Thus, an improved in situ temperature measurement method and apparatus are needed.